Magnetostrictive composites

ABSTRACT

Composite bodies of magnetostrictive materials of the type RE-Fe2, where RE is one or more of the rare earth elements, preferably samarium or terbium, can be suitably hot pressed with a matrix metal selected from the group consisting of aluminum, copper, iron, magnesium or nickel to form durable and machinable magnetostrictive composites still displaying appreciable magnetostrictive strains.

This is a division of application Ser. No. 08/673,550 filed on Jul. 1,1996.

TECHNICAL FIELD

This invention relates to composites of magnetostrictive materials thatcombine appreciable magnetostriction with suitable mechanical propertiesto permit use in aggressive environments such as automotive applicationsand the like. This invention also pertains to methods for themanufacture of such magnetostrictive composites.

BACKGROUND OF THE INVENTION

Magnetostriction occurs when a material on exposure to a magnetic fielddevelops significant strain: at room temperature, sample dimensions canchange by as much as fractions of a percent. Conversely, the strainingof a magnetostrictive material changes its magnetization state.

Magnetostrictive materials have been used with electromagnetic actuatorsto form transducers which serve as, for example, ultrasonic generatorsor fine control valves for the metering of fluids. In theseapplications, variation of the magnetic field is employed to producevarying strains in the magnetostrictive material to produce a mechanicaloutput. Conversely, a suitable magnetostrictive material might beemployed as a torque or force sensor. In fact, such materials are beingconsidered as torque sensors in the form of a magnetostrictive ringmounted on a shaft such as an automobile steering shaft. Torque in sucha shaft would strain the magnetostrictive ring, giving rise to adetectable change in the ring's axial magnetization.

Maximizing device performance naturally suggests using materials havinglarge saturation magnetostriction, λ_(s), which is a dimensionlessmeasure of the field-induced strain frequently expressed in units ofparts per million (ppm). Extremely high values of λ_(s) are found inrare earth-iron compounds such as the terbium-iron compound, TbFe₂,where λ_(s) equals 1750 ppm for a polycrystalline sample. Unfortunately,the rare earth-iron compounds are very brittle materials having littletensile strength, an unpropitious characteristic for automotiveapplications requiring good mechanical properties. On the other hand,stronger and tougher materials such as steels have very limitedmagnetostriction: T250 maraging steel, which is currently beingevaluated in torque sensors, has a λ_(s) of only ˜30 ppm. The wide gulfbetween these two extremes offers ample opportunity and challenge fordeveloping new magnetostrictors combining good magnetostriction withsatisfactory mechanical properties.

The prospect of embedding magnetostrictive powder in a strengtheningmatrix has been sporadically explored as follows.

The Clark and Belson patent, U.S. Pat. No. 4,378,258, entitled"Conversion Between Magnetic Energy and Mechanical Energy," reportedsintering cold-pressed pellets of ErFe₂ with nickel and TbFe₂ with iron.Few details of the properties of these materials were provided. Theyretain some magnetostriction, but as it turns out, the sintered bodiesare brittle and of insufficient strength for many applications such asautomotive sensor applications. Clark and Belson also producedresin-bonded composites of the RE-Fe₂ (RE=rare earth) magnetostrictivecompounds, but these materials also are of insufficient mechanicalstrength for automotive applications.

Peters and Huston of the International Nickel Company attempted toprepare composites of SmFe₂ in nickel by sintering, by extrusion and byhot pressing, but they obtained values of magnetostriction which wereonly modestly larger than that of the nickel alone and did not recommendthe practices. See D. T. Peters and E. L. Huston, "Nickel CompositeMagnetostrictive Material Research for Ultrasonic Transducer," January1977, Naval Electronic Systems Command Contract No. N00039-76-C-0017,U.S. Department of Commerce National Technical Information Service, ADA040336; and D. T. Peters, "Production and Evaluation of ReF(2)-NickelComposite Magnetostrictive Materials," Final Report, January 1979, NavalElectronic Systems Command Contract No. N00039-77-C-0108, U.S.Department of Commerce National Technical Information Service, ADA066947.

Others have also made magnetostrictive composites of RE-Fe₂ materials inepoxy binders. However, none of the above-referenced attempts haveproduced composites of suitable magnetostriction and mechanicalproperties to serve as, for example, torque sensors in demandingenvironments such as automotive applications.

An example of a torque sensor such as might be used in an automotiveapplication is found in I. J. Garshelis, IEEE Trans. Magn. 28, 2202(1992). Garshelis describes a magnetostrictive ring in whichcircumferential magnetization is maintained by a large static hoopstress, the hoop stress also serving to rigidly attach the ring to theshaft carrying the torque. Stresses in the ring associated with thetorque tilt the magnetization away from the circumferential direction.An axial component of magnetization develops and in turn produces amagnetic field in the space around the ring which is detected by amagnetic field intensity sensor; the magnetic field intensity is used tomeasure the torque in the shaft. An example of the application of such ashaft in an automobile, of course, is a steering column shaft. However,the stresses on the magnetostrictive ring can be quite high, and none ofthe above-described magnetostrictive materials provide a desirablecombination of mechanical strength and large magnetostriction.Accordingly, there remains a need for the development of materialssuitable for such applications.

SUMMARY OF THE INVENTION

This invention comprises synthetic, hot pressed composites consistingessentially of a magnetostrictive powder embedded in a metal matrix of asuitably deformable metal such as iron, aluminum, magnesium, nickel orcopper. Magnetostrictive materials of the RE-Fe₂ compounds are preferredbecause of their excellent values of saturation magnetostriction λ_(s).Thus, any of the rare earth elements such as yttrium, lanthanum, cerium,praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium or lutetium or mixtures of them maybe considered suitable. However, SmFe₂ or TbFe₂ are preferred because oftheir high values of magnetostriction. SmFe₂ is especially preferred atthis time because of its relatively low cost and ready availability. Thevalue of λ_(s) for SmFe₂ is -1560 ppm. The negative value ofmagnetostriction means that in contrast to TbFe₂ (λ_(s) =1750 ppm),which expands in the direction of an applied magnetic field, SmFe₂contracts in the field direction and expands laterally. Its relativecontraction (strain) in the dimension parallel to the applied field isgiven by λ.sub.∥ it, which has been measured by the inventors on a purepolycrystalline SmFe₂ ingot to be λ.sub.∥ =-1150 ppm.

Most of the examples in the specification will refer to SmFe₂. However,it is to be recognized that other RE-Fe₂ materials may be employed whereRE may be any desired rare earth element or mixtures of rare earthelements. Single phase RE-Fe₂ powders are preferred. They may beprepared either from annealed ingot or from melt spun ribbons to obtainlow coercivity (H_(ci) ≈0.3 kOe) or high coercivity (H_(ci) ≈3 kOe)magnetostrictive powder, respectively. Consolidation of themagnetostrictive powder with the particulate matrix metal isaccomplished by hot pressing. Preferably, the matrix metal is deformableat temperatures and pressures which permit bulk consolidation by hotpressing. In the present embodiment, the particle size of the matrixmaterial is roughly the same as that of the magnetostrictive material,but it is to be understood that the size distributions of matrixparticles and magnetostrictive particles could be tailored to optimizedensification of the composite. The magnetostrictive powder and matrixpowder are mixed in suitable proportions. Suitably, the matrix powderconstitutes from 20% to 80% by volume of the total composite. Ingeneral, however, a preferred balance of properties is obtained whenroughly 30% to 50% by volume of the magnetostrictive powder is employed.

The powders are uniformly mixed and hot pressed. A preliminary compactof the mixture could be prepared at ambient condition, but the mixtureis ultimately hot pressed at a temperature selected to (1) facilitatethe flow of the matrix material so that it substantially envelopes theparticles of magnetostrictive powder and (2) minimize decomposition ofthe magnetostrictive powder. The duration of the hot pressing operationis preferably just a period of a few minutes, for example, four to 20minutes, a period chosen to minimize alteration or degradation of themagnetostrictive particles. The resultant compacts have very usefulproperties. They retain a very useful proportion of the magnetostrictionof the starting material. They are hard, tough and may even be machinedfor application as a torque sensor ring as described above.

While the invention has been described in terms of a brief summary,other objects and advantages of the invention will become more apparentfrom a detailed description thereof which follows. In this detaileddescription, reference will be had to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph at 400× magnification of a transversesection of a 1/2-inch disc of SmFe₂ ingot particles (50 vol %) in Fematrix, hot pressed at 610° C., 95 MPa pressure.

FIG. 2 is a photomicrograph at 400× magnification of a transversesection of a 1/2-inch disc of SmFe₂ melt-spun ribbon particles (50 vol%) in Fe matrix, hot pressed at 610° C., 95 MPa pressure.

FIG. 3 is a graph of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) as afunction of applied magnetic field H for hot pressed SmFe₂ /Fecomposites made at various SmFe₂ fill fractions. The quantity λ.sub.⊥ isthe strain measured in a direction perpendicular to the applied field.

FIG. 4 is a graph of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) as afunction of applied magnetic field H for hot pressed SmFe₂ /Alcomposites made at various SmFe₂ fill fractions.

FIG. 5 consists of graphs of the dependence of the magnetostriction2/3(λ.sub.∥ -λ.sub.⊥) at H=19 kOe (), the density ρ normalized to thetheoretical density ρ_(theoretical) (□), and the hardness (▾) on thevolume fraction of SmFe₂ in hot pressed composites made with an Fematrix.

FIG. 6 consists of graphs of the dependence of the magnetostriction2/3(λ.sub.∥ -λ.sub.⊥) at H=19 kOe (), the density ρ normalized to thetheoretical density ρ_(theoretical) (□), and the hardness (▾) on thevolume fraction of SmFe₂ in hot pressed composites with an Al matrix.

FIG. 7 consists of graphs of the dependence of the magnetostriction2/3(λ.sub.∥ -λ.sub.⊥) at H=19 kOe (), the density ρ normalized to thetheoretical density ρ_(theoretical) (□), and the hardness (▾) on the hotpress temperature in 50 vol % SmFe₂ composites with Fe.

FIG. 8 consists of graphs of the dependence of the magnetostriction2/3() at H=19 kOe (), the density ρ normalized to the theoreticaldensity ρ_(theoretical) (□), and the hardness (▾) on the hot presstemperature in 50 vol % SmFe₂ composites with Al.

FIG. 9 is a plot of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) in ppm as afunction of applied magnetic field H for hot pressed SmFe₂ /Fecomposites made at 50% fill fractions.

FIG. 10 is a plot of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) in ppm as afunction of applied magnetic field H for hot pressed SmFe₂ /Alcomposites made at 50% fill fractions.

FIG. 11 is a plot of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) in ppm as afunction of applied magnetic field H for hot pressed SmFe₂ /Nicomposites made at 50% fill fractions.

FIG. 12 is a plot of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) in ppm as afunction of applied magnetic field H for hot pressed SmFe₂ /Mgcomposites made at 50% fill fractions.

FIG. 13 is a plot of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥) in ppm as afunction of applied magnetic field H for hot pressed SmFe₂ /Cucomposites made at 50% fill fractions.

DESCRIPTION OF PREFERRED EMBODIMENTS

As summarized above, the composites of this invention are hot pressedRE-Fe₂ /M composites, where RE is one or more of the rare earth metalelements and M is a suitably deformable matrix metal that substantiallypreserves the RE-Fe₂ magnetostrictive compound at hot pressingtemperatures. Suitable examples of the M constituent are aluminum,copper, iron, magnesium and nickel. As stated above, terbium andsamarium are the preferred rare earth constituents because of the highmagnetostriction of their compounds with iron, but SmFe₂ is especiallypreferred because of the ready availability and relatively low cost ofsamarium and used in the following examples for convenience andconsistency of composition for comparison of data.

Also as stated above, the RE-Fe₂ magnetostrictive material may beemployed as an ingot powder or as a melt-spun ribbon powder as will befurther described below.

EXAMPLE OF PREPARATION OF MAGNETOSTRICTIVE POWDERS

SmFe₂ Ingot Powder

Starting ingots having the Sm₁.05 Fe₂ stoichiometry were produced byinduction melting the elemental constituents in an argon atmosphere. Theexcess samarium was introduced to compensate for the loss of samariumdue to its high vapor pressure during melting and subsequent long termanneal. Electron microprobe analysis revealed the as-cast ingot to be amulti-phased mixture of SmFe₂, Sm₂ Fe₁₇, SmFe₃ and elemental Sm. As-castingots have been homogenized by annealing for either 100 hours or 270hours at 700° C. to obtain single phase ingots of SmFe₂. Both annealsgenerated identical single-phased SmFe₂ ingots as determined by x-raydiffraction analysis.

Considerable samarium loss occurs during the heat treatment, and theheat treated ingot consists of a pure SmFe₂ core comprising the bulk ofthe sample and surrounded by a multi-phase skin of samarium-depletedmaterial about one millimeter thick. The skin was removed and theremaining ingot ground to powder by high energy ball milling undermethanol for 10 minutes in a SPEX 8000 mixer/mill to obtain SmFe₂powder. The particle sizes were in the range of about 10 to 250micrometers.

Melt-Spun SmFe₂ Ribbon Powder

Induction melted ingots of SmFe₂ or (SmFe₂)₉₇.5 Al₂.5 were prealloyedfor use in melt spinning. Aluminum was initially incorporated in some ofthe alloys to enhance the formation of amorphous material. It hassubsequently been found that amorphous SmFe₂ can be formed withoutaluminum additions. The SmFe₂ ingots used for melt spinning did notinclude excess samarium. Homogeneity is provided by melt spinningitself, and no long term anneal is required so that there is lesspotential for samarium to be lost during processing. Pieces of the ingotwere induction melted in a quartz crucible under argon inert gasatmosphere and melt spun through a 0.6 mm diameter orifice onto thesurface of a rapidly-spinning, chromium-plated copper quench wheel. Thequench rate was adjusted by changing the velocity v_(s) of the wheel.The diameter of the wheel was about 10 inches.

SmFe₂ ribbons melt spun at v_(s) =20 or 30 m/s were completely amorphousas demonstrated by their powder x-ray diffraction pattern. Differentialscanning calorimetry of v_(s) =30 m/s ribbons revealed thecrystallization temperature to be T_(x) =560° C. for aluminum-freeSmFe₂. After crystallization by heat treatment at 700° C. for 15minutes, the ribbons were nearly single phase SmFe₂ as shown by x-raydiffraction. A few impurity phase peaks were barely detectable,suggesting that the final ribbons may have had a slight samariumdeficiency. Powders were obtained from the ribbons by high energy ballmilling for 10 minutes. The particle sizes were in the range of about 10to 75 micrometers.

The ingot and melt-spun preparation methods are distinguished mainly bythe resultant magnetic properties. Whether amorphous or crystallized,the melt-spun ribbons of SmFe₂ have significant intrinsic coercivityH_(ci) of about 2.4 to 2.6 kOe. In contrast, the ingot-based SmFe₂ hassubstantially lower H_(ci) of about 0.3 kOe.

Synthesis of Hot Pressed Composites

For purposes of illustration, the synthesis of hot pressed composites ofRE-Fe₂ /M will be described in terms of composites of SmFe₂ and eitheraluminum or iron powder. As stated above and as will be exemplifiedbelow, other matrix metals may be employed as well as other rare earthmetal constituents of the magnetostrictive powder.

Powders of SmFe₂ (ingot, amorphous ribbon or crystallized ribbon) wereblended to known volume fractions with either aluminum or iron powderand thoroughly homogenized by hand. The SmFe₂ magnetostrictive powder isessentially black, and the iron or aluminum powder is considerablybrighter. The powders were mixed with a spatula until a uniform graymixture was obtained. Mixtures of various matrix metal fill fractionshave been prepared as described below.

Approximately one cubic centimeter of mixed powder was taken for hotpressing under vacuum in an inductively-heated 12.7 mm ID graphite dieusing tungsten titanium carbide rams. Unheated powder was loaded into anunheated die and supported there by the lower ram. Sufficient powder wasadded to produce cylindrical disks six to seven millimeters in height.The die was equipped with a transversely penetrating thermocouplelocated within 1/8 inch of the internal wall of the die. The output fromthe thermocouple was used to control the heating of the die, whichserved as a susceptor for the induction heating coil.

The inductor was then turned on, and the die and contents were rapidlyheated to a predetermined hot pressing temperature. The hot pressingtemperature is largely dependent upon the selected matrix metal and thedesire to avoid degradation of the rare earth-iron compound orinteractions between the matrix metal and the magnetostrictive powder.Examples of hot pressing temperatures for aluminum and iron matrixmetals are illustrated further below.

In the case, for example, of a SmFe₂ /50 vol % iron mixture to be hotpressed at 610° C., the mixture was heated in the die with no appliedpressure to a temperature of about 520° C. whereupon the rams wereactivated to press forces of about 10 to 12 kN, which translate tocompacting pressures of 79 to 95 MPa. The heating rate was 50° C. perminute. As soon as the temperature of the thermocouple in the dieadjacent the cavity reached 620° C., heating was terminated and theupper ram removed and the lower ram raised to expel the hot pressedcompact from the die cavity. Thus, hot pressing is a relatively fastprocess. The specimen spends five minutes or less at an elevatedtemperature. It is perceived that this practice is superior to sinteringbecause of the much shorter heating time, which provides an advantage inminimizing SmFe₂ decomposition or any undesirable interaction betweenthe matrix metal and the magnetostrictive constituent. The heatingrenders the matrix metal more deformable and enables it to flow aroundthe magnetostrictive powder particles to form a hot pressed compact thatis characterized by particles of the magnetostrictive material in acontinuous matrix. FIGS. 1 and 2 are photomicrographs illustrating crosssections of compacts in which the matrix metal is iron and themagnetostrictive material is SmFe₂ ingot powder (50 vol %) in FIG. 1 andcrystallized SmFe₂ ribbon melt spun at 30 m/s (50 vol %) in FIG. 2. Theejected hot pressed composite is rapidly cooled in a stream of coldnitrogen gas.

FIG. 1 is a photomicrograph at 400× magnification of a portion of atransverse section of a 1/2-inch diameter disc of SmFe₂ ingotparticles/50% Fe hot pressed at 610° C. under 95 MPa pressure. SmFe₂ingot powder particles are seen at 20. The iron matrix is seen at 22.The compact is not fully dense as indicated by the presence of pores 24.It is believed that there may be an indication at 26 of the presence ofa non-magnetostrictive reaction product (i.e., reaction between SmFe₂and iron during hot pressing).

FIG. 2 is a photomicrograph at 400× magnification of a portion of atransverse section of a 1/2-inch diameter disc of SmFe₂ melt-spunparticles/50% Fe composite also hot pressed at 610° C. and 95 MPapressure. SmFe₂ ribbon particles are seen at 20' and the iron matrix at22. Microstructural pores 24 are present. Again, there are indicationsof reaction products 26.

In both FIG. 1 and FIG. 2, a length of one inch (25.4 mm) is equivalentto about 63.5 μm.

Magnetostrictive Properties

Physical properties of SmFe₂ /50 vol % Fe and SmFe₂ /50 vol % aluminumcomposites hot pressed at 610° C. and 540° C., respectively, are shownin Table 1.

                                      TABLE 1                                     __________________________________________________________________________              SmFe.sub.2 /50% Fe (610° C.)                                                       SmFe.sub.2 /50% Al (540° C.)                                                            T250                                             Crystallized                                                                              Amorphous                                                                           Crystallized                                                                             Maraging                                         Ribbons                                                                                Ingot                                                                             Ribbons.sup.a                                                                       Ribbons                                                                             Ingot                                                                                Steel                               __________________________________________________________________________    HD ∥ (H = 19 kOe) (ppm)                                                         -286  -443 -126  -280  -339  29                                    B.sub.r (kG)                                                                                        1.0 4.5                                                                                  1.6                                                                                 0.3                                    H.sub.ci (kOe)                                                                                      0.2.4                                                                                    3.2                                                                                 0.2                                    4(H = 6 kOe) (kG)                                                                          9.7      7.4                                                                                      2.0                                                                                 1.7                                    ρ (g/cm.sup.3)                                                                                  7.0                                                                                4.7                                                                               4.9                                                                                   5.1                                                                                 8.0                              ρ/ρ.sub.theoretical (%)                                                         86               89                                                                               94        92                                    E (GPa)                         646                                                                                        207                              Rockwell hardness                                                                                60 (B)B)                                                                          63 (B)                                                                               ≦49 (B)                                                                     ≦62 (B)                                                                     31 (C)                                __________________________________________________________________________     .sup.a The ribbon composition of this sample was (SmFe.sub.2).sub.97.5        Al.sub.2.5.                                                              

Referring to the above table, the ratio ρ/ρ_(thieoretical) compares themeasured density ρ with the calculated theoretical densityρ_(theoretical) obtained from a weighted average of the density of theconstituents. These values range from 86 to 94 percent of full density,suggesting that significant porosity remains in these specificconsolidated samples. The ratio is, at best, a rough estimate sinceρ_(theoretical) does not take into account the densities of reactionproducts (if any) due to partial decomposition of the SmFe₂.

The hardness values were obtained using a standard hardness tester onthe Rockwell B scale. The Rockwell hardness typically registers around60 Rockwell B. By comparison, the T250 maraging steel is, of course,much harder--about 31 Rockwell C.

Young's modulus, E, for these samples was estimated in compression byapplying a pressure of approximately 1 MPa and measuring the resultingsmall deformations parallel to the applied force with a constantanstrain gauge glued to the cylindrical wall of the disks. For all ofthese samples, E is found to be roughly 60 GPa. This compares with 207GPa for polycrystalline iron and 69 GPa for polycrystalline aluminum.

The magnetic properties were obtained from demagnetization curves. Thenonmagnetic aluminum matrix produces composites whose magnetizationclosely follows that of the parent SmFe₂ material reduced by the volumefraction. The remanence of the ribbon composite is B_(r) =1.6 kG and isvery close to one-half of the B_(r) =3.4 kG value characteristic of thecrystallized ribbons. The coercivity of this sample, H_(ci) =3.2 kOe, isactually significantly larger than the coercivity of the startingribbons, H_(ci) =2.4 kOe; evidently the composite microstructuresupplies additional magnetic hardening. In contrast to the aluminumcomposites, the magnetic iron matrix dominates the magnetization of itscomposites.

In the above table, the component of magnetostriction parallel to theapplied magnetic field, λ.sub.∥, was measured at room temperature usinga constantan strain gauge. The λ.sub.∥ values in Table 1 were measuredin an applied field of 19 kOe and are given in parts per million.

It should be noted that the intent of melt spinning in this invention isto produce magnetically hard SmFe₂ material for inclusion into thecomposite. As Table 1 shows, useful magnetostriction is obtained evenwith amorphous SmFe₂ ribbons; however, crystallized ribbons arepreferred. As in the examples described here, crystalled SmFe₂ ribbonscan be obtained by heat treatment of the amorphous ribbons prior tofabricating a composite. In the case of composites, such as those withFe, where the hot pressing temperature is above the crystallizationtemperature, the heat treatment is not required, since crystallizationwill take place during hot pressing. Finally, those well versed in theart of melt spinning will recognize that crystalline SmFe₂ ribbons canbe formed directly during melt spinning by using suitable wheel speeds.

When all of the data of Table 1 are considered, it is seen that thecomposite magnetostrictors provide an excellent balance in bothmagnetostrictive properties and physical properties. Clearly, thephysical properties are such that these materials can be used inautomotive applications. Furthermore, it has been found that thesematerials are machinable: the 12.7 mm ID disks of 6 to 7 mm height couldbe drilled without breaking the disk. Quarter inch holes were drilledcompletely through the disks, leaving a cylinder or torus that remainedintact and could provide the basis for a force gauge or torsion sensor.

ADDITIONAL EXAMPLES

SmFe₂ Fill Fraction and Hot Press Temperature

An additional series of examples were conducted using annealed,single-phased SmFe₂ ingot. Starting ingots having Sm₀.333 Fe₀.667stoichiometry were cast by induction melting and annealed for 100 hoursto produce SmFe₂ single phase material. The ingots were high energy ballmilled for five minutes to obtain SmFe₂ powder having a particle size of10 to 250 μm. The powder was mixed with either iron or aluminum powderand hot pressed at 95 MPa to obtain a composite cylinder 12.7 mm indiameter and about 7 mm high. A first series of composites was hotpressed at 610° C. (Fe) or 540° C. (Al) in which the volume fraction ofSmFe₂ was varied between 20% and 100%. A second series of composites wasfabricated in which the SmFe₂ fraction was fixed at 50% by volume whilethe hot press temperature was varied between 530° C. and 730° C. foriron and between 500° C. and 620° C. for aluminum.

In these examples, magnetostriction was measured with a techniqueslightly different from the method described above for measuringλ.sub.∥. In this second arrangement, two strain gauges simultaneouslymeasure both the strain λ.sub.∥ parallel to the direction of H and thestrain λ.sub.⊥ perpendicular to H, providing a direct measurement of thedifference (λ.sub.∥ -λ.sub.⊥). In a magnetic field large enough tosaturate the magnetostrictive response, this difference is proportionalto the saturation magnetostriction λ_(s) : λ_(s) =2/3(λ.sub.∥ -λ.sub.⊥).For an isotropic magnetostrictor (as is often assumed to be the case forrandomly oriented polycrystalline materials) λ_(s) =λ.sub.⊥.Magnetostrictions were measured in magnetic fields up to ±19 kOe. Thephysical density ρ was determined from the sample dimensions and weight.The hardness was obtained using a hardness tester on the Rockwell Bscale.

The magnetostrictive strains as functions of the applied magnetic fieldH are shown for various SmFe₂ fill fractions in FIGS. 3 and 4 made withiron and aluminum, respectively. In each test of the composites withdifferent fill fractions, the magnetic field was varied continuouslyfrom +19 kOe to -19 kOe and back to +19 kOe. This accounts for the loopsin each magnetostrictive strain curve. As expected, composites withlarger SmFe₂ fill fractions show higher magnetostrictive strains.

None of the magnetostriction curves in FIGS. 3 and 4 reach saturation inthe maximum applied field of 19 kOe. It becomes increasingly easy tosaturate the magnetostriction as the SmFe₂ content increases, with thepure SmFe₂ sample having the most nearly saturated appearance.

The dependence of λ at 19 kOe on the SmFe₂ fill fraction is shown inFIG. 5 for the composites made with iron. Also shown is the physicaldensity ρ normalized to its theoretical value calculated from the volumefraction of the two constituents and the known densities of the SmFe₂and iron components. The ratio ρ/ρ_(theoretical) serves as a measure ofthe porosity of the sample. Thus, from FIG. 5 it can be discerned thatthe porosity at SinFe₂ fill fractions up to 50% is less than 15% anddecreases as the SmFe₂ fraction drops. At high fill fractions, theporosity is quite large, reaching a value exceeding 30% in a samplecomprised of pure hot SmFe₂ powder. Finally, the hardness is also shownin FIG. 5 as the filled inverted triangles. The pure SmFe₂ sample is toosoft to register on the Rockwell B scale, but the hardness increasessteadily as the SmFe₂ fraction decreases, reaching nearly 90 Rockwell Bat low fill fraction.

FIG. 6 shows similar data for composites made with aluminum. The trendsin magnetostriction and density are very similar to those observed inthe iron composites. The density of the aluminum composites tends to beslightly higher than the corresponding iron composites, probably becauseof the greater deformability of aluminum. The hardness is substantiallyreduced in the aluminum composites and also quite variable. This can beassociated with the mechanical softness of annealed aluminum andsuggests that the hardness of aluminum composites is quite sensitive tothe details of the hot press.

The hot press temperature is varied in FIGS. 7 and 8 for composites madeat 50 vol % SmFe₂ with iron and aluminum, respectively. In both cases,the magnetostriction decreases slowly with increasing hot presstemperature. This loss is ascribed to chemical reaction between theSmFe₂ and the iron or aluminum matrix at elevated temperature whichconverts part of the material at the particle interfaces into additionallow magnetostrictive rare earth-iron or rare earth-aluminum phases.

FIGS. 5 through 8 indicate that the mechanical and magnetostrictiveproperties can be balanced for the requirements of a specificapplication via appropriate selections of fill fraction and hot presstemperature. FIGS. 7 and 8 show that higher hot press temperature leadsto significantly better hardness and higher density, but reducedmagnetostriction. As FIGS. 5 and 6 demonstrate, however, the latter canbe at least partially compensated by increasing the fill fraction of themagnetostrictive component.

It is to be recognized that the magnetostriction data summarized inFIGS. 7 and 8 were obtained on hot pressed composites that hadexperienced hot pressing temperatures for only 10 minutes or less. It isbelieved that retention of relatively high magnetostriction in all ofthe subject samples is due to preventing unnecessary prolonged exposureof the composite mixtures to high temperatures during pressing.

SmFe₂ Composites with Copper, Nickel or Magnesium Matrix Metal

A series of composite specimens was prepared using SmFe₂ particles withmatrix metals other than iron and aluminum. The practice of thisinvention is also demonstrated using copper, magnesium or nickel as thematrix material. Since it has been demonstrated that composites of equalparts by volume of RE-Fe₂ magnetostrictor and matrix metal provide avery suitable balance of magnetostriction and mechanical properties,such composites were chosen for the following examples.

The specimens were prepared using either SmFe₂ ingot particles orcrystallized melt-spun ribbon particles to make 50% SmFe₂ fill ratiocomposite with copper or magnesium or nickel. The respective forms ofSmFe₂ particles were prepared as described above. These magnetostrictiveparticles were mixed with elemental copper or magnesium or nickelparticles and hot pressed as described above into cylindrical discs of12.7 mm diameter and 7 mm long. The copper matrix composites werepressed at 470° C. The nickel composites were pressed at 610° C. and themagnesium composites at 575° C. to 610° C. The composites were allpressed at 95 MPa and remained at their respective hot presstemperatures for no more than 10 minutes. Like samples with iron andaluminum as matrix metal were prepared for direct comparison ofmagnetostrictive values.

FIGS. 9 through 13 are graphs of magnetostriction 2/3(λ.sub.∥ -λ.sub.⊥)in ppm for the iron matrix, aluminum matrix, nickel matrix, magnesiummatrix and copper matrix composites, respectively. In each example, thecomposites were subjected to an applied magnetic field that was variedfrom +19 kOe to -19 kOe and back to +19 kOe. Like magnetostriction datafor T250 maraging steel is included on each plot for comparison. It isagain observed that the magnetostriction of maraging steel is positive,although quite small, while that of the SmFe₂ -containing composites isnegative. The maraging steel magnetostriction curve reaches saturationat less than H=5 kOe while the RE-Fe₂ samples do not reach saturation inan applied field of 19 kOe. From the displacement of the peaks in themagnetostriction curves from H=0 in FIGS. 9-13, it is evident that thecomposites containing melt-spun SmFe₂ particles possess appreciablecoercivity which is not present in the SmFe₂ ingot powders or in themaraging steel samples.

The copper matrix, magnesium matrix and nickel matrix composites hadmechanical properties comparable to the aluminum and iron matrixcomposites described above. Thus, it is clear that this inventionprovides a family of useful magnetostrictive composites that offers abalance of high magnetostriction and suitable mechanical properties. Thesubject composites have sufficient strength and hardness such that theycan be drilled or otherwise mechanically worked and they can surviveaggressive environments or applications such as are encountered inautomobile and truck components. Furthermore, these composites canprovide suitable magnetostriction for use in sensor or transducerapplications on such vehicles. When the magnetostrictive intermetalliccompound phase is initially prepared as suitably crystallized, melt-spunribbon particles, such particles and the resulting composite displaymagnetic coercivity.

An important feature of the processing aspect of this invention is thatthe duration of the hot pressing step be managed and limited so as toobtain suitable strength and density without unacceptable degradation ofthe magnetostriction. In general, it is preferred that the time that thecomposite is held at its hot pressing temperature be no more than about20 minutes.

While this invention has been described in terms of certain preferredembodiments thereof, it will be appreciated that other forms couldreadily be adapted by one skilled in the art. Accordingly, the scope ofthis invention is to be considered limited only by the following claims.

We claim:
 1. A magnetostrictive composite body consisting essentially ofdiscrete particles of magnetostrictive material selected from the groupconsisting of (i) substantially single phase particles of themagnetostrictive intermetallic compound RE-Fe₂, where RE is one or morerare earth elements, and/or (ii) amorphous particles of correspondingelemental composition, embedded in a strengthening metal matrix phaseformed by hot mechanical compaction and deformation of particles of adeformable metal, the density of said composite being 70% or more of itstheoretical density based on the actual densities of said deformablemetal and said magnetostrictive material and the magnitude of themagnetostriction |2/3(λ.sub.∥ -λ.sub.⊥)| is greater than 85 ppm in anapplied magnetic field of 19 kOe.
 2. A magnetostrictive composite bodyas recited in claim 1 in which RE comprises samarium or terbium.
 3. Amagnetostrictive composite body as recited in claim 1 in which saiddeformable metal is selected from the group consisting of aluminum,copper, iron, magnesium and nickel.
 4. A magnetostrictive composite bodyas recited in claim 1 in which said magnetostrictive composite comprisesmagnetically coercive RE-Fe₂ particles made by crystallizing melt-spunribbons of said composition.
 5. A magnetostrictive composite body asrecited in claim 1 in which the deformable metal comprises aluminum, andthe composite has a magnetostriction |2/3(λ.sub.∥ -λ.sub.⊥)| in excessof 100 ppm in magnitude in an applied magnetic field of 19 kOe.
 6. Amagnetostrictive composite body as recited in claim 5 in which saidmagnetostrictive composite comprises magnetically coercive RE-Fe₂particles made by crystallizing melt-spun ribbons of said composition.7. A magnetostrictive composite body as recited in any of claims 1through 6 in which said body contains 30% to 80% by volume of saiddiscrete particles of magnetostrictive material.